1. Field of the Invention
The present invention generally relates to the field of electrical cables. In particular, the present invention relates to an electrical cable resistant to fire, water and mechanical stresses.
2. Description of the Related Art
As is known, an electrical cable resistant to fire (known as a “fire resistant” cable) is a cable configured so as to be capable of continuing to function with acceptable electrical performance even if, owing to a fire, it is exposed to a naked flame for a period of time, at temperatures of up to 800° C.-900° C. or above.
Fire resistant electrical cables are used for various purposes including applications on ships or marine platforms. In these applications, it is necessary to ensure that certain electrical systems (such as lighting, sprinklers, bulkhead sealing systems and the like) will continue to operate for a given period after the outbreak of a fire, as specified, for example, in the SOLAS “Safe Return to Port” regulation, Chapter II-2/21.4. This makes it possible to extinguish the fire, to evacuate the area safely, and to return the ship to port if necessary.
To ensure correct operation, as well as fire resistance, the electrical cables must also have high impermeability to the water which is emitted by the fire-fighting systems (such as sprinklers and hydrants) and/or which results from any flooding. The electrical cables must also be capable of maintaining their characteristics of fire resistance and impermeability to water even in the presence of the intense mechanical stresses (such as vibration, impact, compression and the like) to which they are typically subjected during the fire extinguishing and evacuation operations.
EP 1 798 737 relates to flame resistant electric cables which, as subjected to a direct flame and a consequent temperature variation from 750° C. to 930° C., provide a safe current flow for a time period from 1 to 2 hours. In the cable, each individual wire is encompassed by a glass fibre strip onto which a mica layer has been glued. Outside this there are provided a layer thread braid impregnated by a polyurethane resin and an insulating elastomeric layer. The joined conductors are wrapped by a combined copper strip and a glass fibre strip to provide waterproof and insulating properties. For completing the cable it is finally coated by an outer coating layer of thermoplastic or elastomeric material.
U.S. Pat. No. 5,707,774 relates to a flame resistant electric cable which is capable of resisting temperatures in the neighborhood of 1000° C. for at least two hours. The cable comprises conductors, each of which is surrounded by an insulating layer of silicone rubber and a layer of braided inorganic material such as silica or ceramic. Outside the conductors, a further layer of aluminium/polyethylene terephthalate, a layer of silicone rubber, and an outer braided jacket, made of fiberglass material, are provided.
FR 2 573 910 describes a coating capable of protecting a cable from flames and dielectric breaks when subjected to temperatures of 800° C.-1000° C. for more than 15 minutes. The coating is said to be not fire propagating, resistant to shocks, vibrations and water jets. The coating comprises two or more mica layers, one layer of polymer resin optionally charged with refractory inorganic particles, one layer of braided glass fibres impregnated with polymer resin optionally charged with refractory inorganic particles, and one outer sheath of polymer resin optionally charged with refractory inorganic particles.
GB 1582580 describes a fire resistant cable comprising two pairs, each of which is coated by a mica layer, an insulating layer comprising heat resistant rubber, a layer of thermoplastic elastomer filled with aluminium hydroxide, a glass fibre layer, a braided metal armour and an outer sheathing of polyethylene or polypropylene material. The cable is said to resist temperatures of 650-1100° C. for more than 30 minutes, and, subsequently, to resist vibrations.
US 2002/0046871 describes a fire resistant electrical cable comprising a metallic conductor, a first glass and/or mica containing layer wound with an overlap of 50%, a second glass and/or mica containing layer also wound with an overlap of 50%, and an insulating layer of plastic material, for example polyethylene, with flame-retardant additives.
The Applicant has observed that none of the electrical cables described above is concurrently fire resistant and impermeable to water in the presence of mechanical stresses.
In particular, in the electrical cable described in EP 1 798 737, the combination of copper strip and glass fibre strip does not provide sufficient protection against mechanical stresses—which can be exceptionally intense—which an electrical cable may be subjected to during rescue operations following a fire. Furthermore, in the presence of flames, the elastomeric insulating layer softens as a result of the rise in temperature, or even burns. The softened material or the ashes produced by combustion are collapsible and cannot support the outer layers of the cable, which undergo structural collapse. The latter are therefore no longer capable of providing impermeability to water and adequate protection against mechanical stresses.
In the cable described in U.S. Pat. No. 5,707,774, the silicone rubber layers enclosing the conductors also soften when the temperature rises and are combustible. In the presence of flames, therefore, they would give rise to the same problems as those mentioned above. On the other hand, the outer layers of this cable (the silicone rubber layer and the outer braided jacket made of fiberglass material) do not provide either impermeability to water or adequate protection against mechanical stresses in case of flame.
Regarding the cable described in FR 2 573 910, the outer sheath of polymer resin, optionally charged with refractory inorganic particles, ceramifies in the presence of flames, producing a residue which, although fire resistant, cannot provide adequate protection against direct mechanical stresses, as a result of which it may become water-permeable.
In the cable described in GB 1582580, the thermoplastic elastomeric layer also softens when the temperature rises, and is combustible. In the presence of flames, therefore, it would give rise to the same problems as those mentioned above. On the other hand, the outer layers of this cable (glass fibre layer, braided metal armour and outer sheathing of polyethylene or polypropylene material) do not provide impermeability to water in the presence of flames.
Regarding the electrical cable described in US 2002/0046871, its outer layer of plastic material (such as polyethylene, with flame-retardant additives) cannot impart impermeability to water to the cable in the presence of flames.